app-ui-design

App UI Design

This skill guides UI/UX in generated apps (Fusebase Apps). Use shadcn/ui for all UI. Apply a clear design direction and avoid generic AI aesthetics.

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Design philosophy

Clarity over decoration. Every visual choice supports hierarchy and scannability. Prefer one bold aesthetic direction (minimal, warm, editorial, etc.) and execute it consistently.

• Purpose first: What does the screen do? Who uses it?
• Tone: Choose one direction (e.g. minimal, warm/SaaS, editorial, utilitarian) and stick to it.
• Differentiation: Avoid default "AI" look: no Inter/Roboto-only, no purple gradients on white, no same-as-everyone layouts. Vary fonts, palette, and density per context.

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Visual identity

Tailwind CSS v4

Apps use Tailwind CSS v4 (via @tailwindcss/postcss). Key differences from v3:

• Import: Use @import "tailwindcss" in globals.css (not @tailwind base/components/utilities).
• No tailwind.config.js: Configuration is CSS-first. Use @theme in CSS to define custom tokens.
• Content detection is automatic: Tailwind v4 scans project files automatically. If needed, use @source "../path/**/*.tsx" to add extra directories.

⚠️ CRITICAL — CSS variables in arbitrary values:

Do NOT define raw :root CSS variables and reference them via arbitrary value syntax. This is the most common Tailwind v4 pitfall:

/* ❌ BROKEN — variables are NOT available to Tailwind's arbitrary value resolver */
:root {
  --card: #ffffff;
  --foreground: #0f172a;
}

// ❌ BROKEN — these classes compile but resolve to empty/broken values
className="bg-[var(--card)] text-[var(--foreground)]"

Instead, use one of these approaches:

1. Use Tailwind's built-in color palette (preferred for most apps):

   // ✅ Works — uses Tailwind's first-class utility classes
   className="bg-white text-slate-900 border-slate-200"
   className="text-indigo-500 bg-indigo-50"
   

2. Register custom tokens via @theme (only if you need custom colors):

   @import "tailwindcss";
   @theme {
     --color-brand: #6366f1;
     --color-surface: #ffffff;
   }
   

   // ✅ Works — registered via @theme
   className="bg-surface text-brand"
   

3. Use @apply for reusable component classes (buttons, inputs):

   .btn-primary {
     @apply inline-flex items-center gap-2 px-5 py-2.5 text-sm font-semibold text-white rounded-xl;
     background: linear-gradient(135deg, #6366f1 0%, #4f46e5 100%);
   }
   

4. Use inline style only for truly dynamic values (e.g. colors from API data):

   // ✅ OK — color is runtime data, can't be a utility class
   style={{ color: product.categoryColor, backgroundColor: `${product.categoryColor}18` }}
   

shadcn/ui (when used)

• shadcn/ui components live in components/ui/ (copied into the project, not installed as a package). Import from @/components/ui/....
• When shadcn/ui is set up, its CSS variables (--background, --foreground, etc.) are registered through its own theme system and work with utility classes like bg-background, text-foreground.
• Prefer variant and size props on shadcn/ui components (e.g. <Button variant="outline" size="sm">) over ad-hoc Tailwind color overrides.
• Use the cn() utility (from @/lib/utils) to merge Tailwind classes safely: cn("base-class", conditionalClass).

⚠️ CRITICAL — Do NOT add a global * CSS reset:

Do NOT add * { margin: 0; padding: 0; box-sizing: border-box; } in globals.css. Tailwind v4's Preflight already applies proper resets. A manual * reset has equal specificity to Tailwind's utility classes and will override them when it appears later in the cascade, breaking padding (p-4, p-5, px-6, etc.) and margin utilities silently.

/* ❌ BROKEN — overrides Tailwind utilities like p-5, m-4 */
* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

/* ✅ CORRECT — Tailwind v4 Preflight handles resets automatically */
@import "tailwindcss";
/* No manual * reset needed */

General Tailwind usage

• Typography: Use Tailwind's typography scale (text-sm, text-base, text-lg, font-semibold, etc.) and keep heading/body consistent throughout the app.
• Radius: Use consistent rounding from the theme (rounded-md for cards and inputs; rounded-full for pills/avatars).
• Colors: Choose ONE approach based on whether shadcn/ui theming is set up:
  • With shadcn/ui: Use its semantic tokens (bg-background, text-foreground, bg-muted, text-muted-foreground, border) — these are pre-registered and work out of the box.
  • Without shadcn/ui: Use Tailwind's built-in palette (bg-white, text-slate-900, bg-slate-100, text-slate-500, border-slate-200).
  • Never mix: Don't use bg-background in an app without shadcn/ui theming — it won't resolve. Reserve inline style for dynamic/computed colors only.

Hierarchy

Establish clear text hierarchy using the same approach as Colors above:

Level	With shadcn/ui	Without shadcn/ui
Primary (main content)	text-foreground	text-slate-900 dark:text-slate-100
Secondary (labels, metadata)	text-muted-foreground	text-slate-500 dark:text-slate-400
Tertiary (timestamps, hints)	Smaller size + text-muted-foreground	Smaller size + text-slate-400

Reserve accent/brand color for CTAs and key UI, not body text.

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UX principles

• Fast input: Optimize tab order, use presets or shortcuts where it fits, combine related steps (e.g. create + add in one flow).
• Feedback: Update UI right after mutations (invalidate queries, toasts for destructive or important actions). Use disabled + a spinner on Button during submits.
• Low cognitive load: Few, clear actions per screen; constrained choices (e.g. fixed categories with icons); avoid wizards when a single form is enough.
• Empty and loading: Always handle empty data (centered message + short guidance) and loading (Skeleton sized to final content, or a Loader2 spinner from Lucide).

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Layout and spacing

• Spacing scale: Use Tailwind spacing consistently (e.g. gap-2/p-2 for inline, gap-4/p-4 for cards/sections, gap-6/py-6 for page rhythm).
• Responsive: Use Tailwind breakpoints (sm:, md:, lg:). Single column on mobile; sidebars/panels as toggles or Sheets (<Sheet>) on small screens. Use flex-wrap and min-w-0 to avoid overflow.
• Content width: Constrain main content (e.g. max-w-2xl mx-auto) for readability on wide viewports.

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Component patterns

• shadcn/ui first: Use shadcn/ui primitives for actions, forms, and feedback: Button, Input, Card, Dialog, DropdownMenu, Select, Textarea, Badge, Skeleton, etc. Do not replace them with raw HTML for interactive elements.
• Forms: Use react-hook-form with zod for validation. Wrap inputs in <FormField>, <FormItem>, <FormLabel>, <FormControl>, <FormMessage> from @/components/ui/form.
• Toast / notifications: Use sonner (toast.success(...), toast.error(...)) or shadcn/ui's useToast hook. Prefer sonner for simplicity.
• Composition: Pass data and callbacks into components (e.g. onSubmit, onClose), not big config objects.
• Icons: Use Lucide React (lucide-react) consistently throughout the app. Apply text-muted-foreground or contextual color classes for icon meaning (e.g. status, category).
• Loading: Add disabled to the button and show a <Loader2 className="animate-spin" /> icon inside it while a request is in flight; use <Skeleton> for content placeholders.

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State and feedback

• Cursor: Always add cursor-pointer to interactive elements (buttons, links, clickable cards) including shadcn/ui Button. For custom <button> elements or <div onClick> handlers, always include cursor-pointer in the Tailwind class list. For disabled states use cursor-not-allowed (and remove cursor-pointer).
• Hover/active: Rely on shadcn/ui component variants and Tailwind hover: utilities; avoid overriding styles on interactive elements without a clear reason.
• Errors: Show inline validation messages via <FormMessage> and a toast or inline alert for API errors. See handling-authentication-errors for 401/token expiry.
• Success: Toast or brief inline confirmation for saves and destructive actions.

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Accessibility

• Use semantic structure: headings, <label> for inputs, landmarks where relevant.
• shadcn/ui components are built on Radix UI primitives — keyboard navigation and ARIA attributes are handled automatically; do not override or remove them.
• Use <FormLabel> for all form inputs so labels are always associated.
• Keep contrast in mind when picking palettes (CSS variable tokens help ensure consistency).

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Dark mode

With shadcn/ui: Dark mode is toggled by adding/removing the dark class on <html>. Use CSS variable-based tokens (bg-background, text-foreground, etc.) which are registered through shadcn/ui's theme system.

Without shadcn/ui: Use Tailwind's dark: variant with the built-in palette: bg-white dark:bg-slate-900, text-slate-900 dark:text-slate-100. Do NOT create raw :root CSS variables and reference them via bg-[var(--name)] — this does not work in Tailwind v4 (see Tailwind CSS v4 section above).

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References

• shadcn/ui: https://ui.shadcn.com — components, theming, and CLI usage.
• Radix UI: Underlying primitive library providing accessible behavior for shadcn/ui components.
• Lucide React: https://lucide.dev — icon set to use consistently.
• AGENTS.md: Use shadcn/ui for app UIs; auth and SDK usage are described there and in fusebase-dashboards, handling-authentication-errors.